Ordinary inserts of this type, in particular spark plugs and glow plugs, display as the securing means an external thread and an external hexagon. The associated boring in the cylinder head for accommodating the insert is provided with a corresponding internal thread, in order that the insert be capable of being screwed into the boring. Therefore, securing is accomplished by thread engagement, whereby used for screwing in the insert and tightening it is an open-end hex wrench that is placed on the external hexagon of the insert.
Installation of an insert of this type is laborious and difficult, as it has turned out to be in many cases. The insert needs to be drawn up tight with a certain torque. If it is seated too loosely, complete sealing between the insert and the cylinder head with the aid of the sealing means is not guaranteed. If the insert is tightened with too great a torque, there then exists the danger of damaging the thread, whereby the insert becomes unusable, and the cylinder head can be damaged. This latter is particularly critical, since the function of the cylinder head can be endangered by this, and a new internal thread must be re-cut in the boring. Therefore, in order to be able to tighten the insert with the correct torque, required is a torque wrench, which, however, is available in the most seldom of cases, and in the case of the owner of the vehicle is generally not available at all. As for the rest, use of an open-end wrench and, in particular, of a torque wrench, which because of the torque-setting arrangement has a larger structural form than a normal socket wrench, is particularly difficult under narrowed space conditions, such as they are frequently to be encountered in the engine space of a vehicle. A further disadvantage is the expense required for the manufacturers of inserts and of internal combustion engines to form on the insert the external thread and the external hexagon, and to cut the corresponding internal threads in the boring in the cylinder head. Additionally required are work steps that increase the cost of producing the inserts and the cylinder heads of the internal combustion engines.